Dentists and other medical workers, when performing certain treatments on a patient's tooth, use endodontic instruments such as endodontic files. These treatments include root canal treatments and other treatments involving the tooth pulp or the root of the tooth. Endodontic instruments may be coupled to a device that rotates the instrument to assist with shaping and/or cleaning the portion of the tooth being treated. These instruments can be manufactured in different sizes with varying amounts of taper applied to the instrument. In typical instruments, lengths range from 20-35 mm (millimeters) and instrument taper ranges from 2% to 12%.
Endodontic instruments are typically manufactured using metal, such as stainless steel or a metal alloy. One type of metal alloy used in manufacturing endodontic instruments is a nickel-titanium (NiTi) alloy. In general, nickel-titanium endodontic instruments provide greater flexibility and are more resistant to cyclic fatigue than stainless steel instruments. However, nickel-titanium endodontic instruments operated in a rotational manner suffer from at least two types of fractures: fracture caused by torsion and fracture caused by flexural fatigue. A torsion fracture occurs when an instrument tip or another part of the instrument is locked in a tooth canal while the shank of the instrument continues to rotate.
Fracture caused by flexural fatigue occurs when the endodontic instrument rotates freely in a curved orientation, which generates tension/compression cycles at the point of maximum flex. For example, as the instrument is held in a static position and continues to rotate, the portion of the instrument shaft on the outside of the curve is in tension while the portion of the instrument shaft on the inside of the curve is in compression. This repeated tension-compression cycle caused by rotation within curved tooth canals increases cyclic fatigue over time and contributes to instrument fracture.
Additional factors that contribute to a failure of endodontic instruments produced using nickel-titanium include the machining and grinding procedures applied during the manufacturing process. These procedures may result in work-hardened areas of the instrument that are brittle. Traditional machining procedures may also result in cracks and tool marks that initiate fractures or otherwise contribute to the failure of the endodontic instrument. In particular, cracks, tool marks and other surface irregularities may induce failure due to the concentration of stress at those irregularities.